A nuclear chlorination reaction of chlorobenzene which is a step for producing p-dichlorobenzene is classified in an electrophilic substitution reaction exhibiting ortho-para orientation, which is known to be chlorinated by a chlorine molecule using a Lewis acid as a catalyst.
Cram & Hammond: “Organic Chemistry” 2nd edition, page 218 (1964) discloses aluminum chloride and ferric chloride as important Lewis acid. Moreover, in page 427, it is disclosed that one halogen atom of a halogen molecule combines with ferric halide or aluminum halide and the other halogen atom becomes cationic halogen to enhance electrophilic property.
Regarding 10-substituted-10H-phenothiazine which is used as a promoter together with the Lewis acid, there are following proposals in order to enhance the para-selectivity upon preparing p-chlorobenzene by nuclear chlorination of benzene and/or chlorobenzene using chlorine molecules and Lewis acid.
EP126669 (corresponding with Japanese Patent Application Laid-open SHO 59-206051) discloses 10-arylcarbonyl-10H-phenothiazine, 10-halocarbonyl-10H-phenothiazine and 10-CHxX1yCO-10H-phenothiazine (wherein X1 represents a chlorine atom or bromine atom, x is 0–2 and y is 1–3). EP474074 (corresponding with Japanese Patent Application Laid-open HEI 4-305544) discloses 10-CF3(CF2)nCO-10H-phenothiazine (n=0, 1 or 2). WO97/43041 discloses 10H-phenothiazine-10-carboxylic acid phenyl esters.
EP126669 does not disclose aluminum compounds as the Lewis acid, but ferric chloride and antimony trichloride are used in Examples. It does not suggest that the aluminum chloride shows an effect superior to ferric chloride and antimony trichloride. EP474074 and WO97/43041 disclose aluminum chloride as the Lewis acid, but only ferric chloride is used in Examples. These references do not suggest that the aluminum chloride exhibits an effect superior to ferric chloride.
Although all the process disclosed in these three patent documents relates to the same proposals for improving the para-selectivity, there are still problems to be improved with respect to reaction time.
Japanese Patent Application Laid-open SHO 60-125251 discloses in Example 3 chlorination of toluene using a Lewis acid together with 10-trifluoroacetyl-phenothiazine as a catalyst system for improving the para-selectivity in nuclear chlorination of alkylbenzene. Table 1 in said reference however shows the fact that aluminum chloride has the para-selectivity inferor to that of ferric chloride and antimony trichloride.